The present invention relates to a power transfer system for controlling the distribution of drive torque between the front and rear wheels of a four-wheel drive vehicle as a function of various system and operator-initiated inputs.
In view of increased consumer popularity in four-wheel drive vehicles, a plethora of power transfer systems are currently being utilized in vehicular driveline applications for selectively directing power (i.e., drive torque) to the wheels of the vehicle. In many power transfer systems, a part-time transfer case is incorporated into the driveline and is normally operable in a two-wheel drive mode for delivering drive torque to the driven wheels. The part-time transfer case also includes a mode shift mechanism which can be selectively actuated by the vehicle operator for rigidly coupling the non-driven wheels to the driven wheels for establishing a part-time four-wheel drive mode. Alternatively, some power transfer systems automatically direct power to the non-driven wheels, without any input or action on the part of the vehicle operator, when traction is lost at the driven wheels. This "on-demand" feature is incorporated into the transfer case by replacing the mechanically-actuated mode shift mechanism with a clutch assembly that transfers drive torque to the non-driven wheels when a low traction condition occurs at the driven wheels. One example of an "on-demand" power transfer system is disclosed in U.S. Pat. No. 4,773,500 to Naito, et al wherein a hydraulically-actuated clutch assembly is operable for automatically controlling the amount of drive torque transferred to the non-driven wheels as a function of the wheel speed difference (i.e., the wheel slip) between the front and rear wheels.
As a further alternative, some vehicles are equipped with full-time transfer cases having a center (i.e., interaxle) differential for splitting the drive torque between the front and rear drivelines. To minimize loss of traction due to wheel slippage, most full-time transfer cases are also equipped with a slip limiting device for locking the interaxle differential to prevent excessive speed differentiation. Examples of manually-actuated differential lock-up arrangements are disclosed in commonly owned U.S. Pat. No. 3,848,691 to Dolan and U.S. Pat. No. 4,677,873 to Eastman. An automatic differential lock-up arrangement is disclosed in commonly owned U.S. Pat. No. 3,845,671 to Sharp et al. wherein an electrically-controlled clutch is actuated to lock-up the interaxle differential when speed differentiation due to a wheel slip condition is detected which exceeds a predetermined value. In addition, torque-biasing differential lock-up arrangements are disclosed in commonly owned U.S. Pat. No. 4,031,780 to Dolan et al. and U.S. Pat. No. 5,046,998 to Frost, which both utilize a viscous coupling to progressively modify the torque distribution in proportion to the magnitude of the speed differentiation across the interaxle differential. Finally, electronically-controlled full-time transfer cases are disclosed in U.S. Pat. No. 4,718,303 to Fogelbert and U.S. Pat. No. 4,860,612 to Dick et al. wherein an electromagnetic biasing clutch is provided across the interaxle differential to controllably bias the torque delivered to the front and rear drivelines in response to wheel slip. While conventional part-time, on-demand and full-time transfer cases are satisfactory to perform their intended function, it is understood that a need exists to develop alternatives which incorporate the various advantages of each into a common transfer case.